A conventional casing 10 kitchen range hood is shown in FIG. 1 and generally includes a main plate 11 which has a hole for connection a pipe, two side plates 12 which are connected to two ends of the main plate 11 by way of welding or riveting along flanges on the periphery of the side plates 12, and a rear plate 13 which is connected to the main plate 11 and the two side plates 12 by the same way as the connection of the side plates 12 and the main plate 11. Each of the main plate 11, the two side plates 12 and the rear plate 13 are made separately and the respective flanges are made so as to connect to each other. The manufacturers have to spend a lot of time to cut, punch, bend and weld these separate items. Of more importance is that the convention casing 10 involves many angled or narrow corners or areas and splits which stock oil particles and are difficult to clean.